Fully support filtering packages, and specifying specific package to filter by name

When querying packages and filtering using the --filter option, we currently only filter when we print the results. Later, however, we end up printing statistics for the full vector of packages. This is confusing behavior -- a user that's intentionally filtering by specific packages probably only cares about those packages. Let's instead add logic to prune the vector of packages so that the packages passed to print_results() are the ones we actually want to count and print.

In a subsequent commit, we also add support for filtering by name.

Signed-off-by: David Vernet void@manifault.com